The present invention relates to a method for measuring an S/N (signal to noise ratio), a C/N (carrier wave level to specific frequency noise level), a third order distortion and a fifth order distortion in an input signal etc. using a spectrum analyzer.
FIG. 1 shows a general configuration example of a spectrum analyzer. An output signal of an object 11 is inputted to a spectrum analyzer 15 as an input signal. In the spectrum analyzer 15, an input signal is supplied to a frequency mixer 17 via an input variable attenuator 16 and then the frequency of the input signal is mixed with the frequency of a local signal from a frequency sweep generator 18. Then the mixed output is supplied to a band pass filter 19 and the output of the filter 19 is amplified by an amplifier 21 and then the frequency of the amplified output is mixed with the frequency of a local signal from a local oscillator 23 by a frequency mixer 22. And then, the intermediate frequency signal is taken out by a band pass filter 24 and the output of the filter 24 is detected by a detector 26. The detected output is converted into a digital signal by an AID converter 28 after passing through a low pass filter 27 and is stored in a buffer memory 29. A control part 31 is a so called CPU and performs a setting of attenuation amount for the attenuator 16 in accordance with a parameter set by parameter setting means 32, a control of the frequency sweep generator 18 by controlling a ramp voltage generator 34 through a timing controller 33. That is, the control part 31 (CPU) performs a setting of a sweep frequency band, a setting of a band width RBW for each of the filters 19 and 24, a setting of a band width VBW for the filter 27 and a setting of a sampling period for the A/D converter 28, as well as a display control of the data stored in the buffer memory 29 on a display unit 35.
In a conventional system, for example when a C/N of a continuous wave input signal is measured, a carrier wave frequency of a signal to be measured and a noise frequency f.sub.N (is usually prescribed in accordance with, for example, a modulation mode of an input signal) whose noise level is to be measured against the signal are set by the parameter setting means 32. Then data are taken out from the memory 29 and are displayed on the screen of the display unit 35 as shown in, for example, FIG. 2A. In addition, a ratio L.sub.C /L.sub.N of a carrier wave frequency data (level) L.sub.C to a data (level) L.sub.N of the noise frequency is displayed on a part of the screen like, for example, C/N--53 dBc/Hz. In this case, since the noise level L.sub.N changes at random, the band width VBW of the low pass filter 27 is usually set to relatively narrow band, i.e., to a level of 1/10+L of the band width RBW of the band pass filters 19 and 24 so that the measured noise levels are averaged.
In the case where a C/N of an input signal is measured using this spectrum analyzer, the following operations are performed in conventional system.
1. A central frequency is set by pushing a frequency button. PA0 2. A frequency interval (an offset value) between a signal to be measured (a carrier wave) and a noise is set. PA0 3. A frequency span (a frequency interval to be displayed on a display screen) is set by pushing a frequency span button. PA0 4. A peak of the carrier wave is coincided with the central frequency of the screen (FIG. 2A). PA0 5. The carrier wave level is coincided with a reference level. PA0 6. A marker is put on a peak point of the signal through a peak search process. PA0 7. A delta marker is used as a marker. PA0 8. The delta marker 38 is coincided with the noise frequency to be measured (FIG. 2A). PA0 9. A noise measurement is selected. PA0 10. An indicated value of a noise level is read. PA0 1. A central frequency is set to the frequency of one of the input signals, namely, the frequency of one of the two fundamental waves on which a intermodulation distortion is based, i.e., a signal having a frequency f.sub.1 and a signal having a frequency f.sub.2. PA0 2. An input signal frequency range, i.e., a displayed frequency range is manually set by pushing a span button. PA0 3. A resolution band width is set by pushing a band width button. That is, the setting of each band width RBW of the band pass filters 19 and 24 is switched from an automatic operation to a manual operation, and then a resolution band width RBW is set. The reason for the manual operation is that when a resolution band width RBW is automatically set, a spectrum of a mutual intermodulation is hidden at the bottom portion of the input signal due to a low level of a intermodulation distortion or an influence of the band pass filters 19 and 24. In such a case, a spectrum of a intermodulation distortion may not be observed. PA0 4. A third order distortion is measured by pushing a TOI button. PA0 5. The above operations 1-3 are repeated by changing the setting of the resolution band width RBW until four clear peaks appear on the display screen, i.e., as shown, for example, in FIG. 2B, until four clear peaks, the spectrums 41 and 42 of two input signals (fundamental waves) having frequencies f.sub.1 and f.sub.2 respectively, a third order distortion 43 of a frequency (2f.sub.1 -f.sub.2) generated by a intermodulation of these two input signals and a third order distortion 44 of a frequency (2f.sub.2 -f.sub.1) appear on the display screen.
In these operations, when a frequency span is set, a band width RBW (usually, the settable width is predetermined) of the band pass filters 19 and 24 is set by trial and error so that the wave forms of the signal portion and the noise portion are accurately displayed.
When a intermodulation distortion is measured by a spectrum analyzer 15, as indicated by a dashed line in FIG. 1, test signals of the same amplitude having frequencies f.sub.1 and f.sub.2 from signal generators 12 and 13 respectively are combined by a power combiner 14 and are supplied to the object 11, and then a intermodulation distortion generated by the object 11 is measured.
Formerly, the measurement of a intermodulation distortion has been performed in the following sequence.
Incidentally, when a level of the fundamental waves 41 and 42 at the input side of the input attenuator 16 is L as shown in FIG. 3A and a level difference between the third order distortions 43, 44 and the respective fundamental waves 41, 42 is .DELTA.L, a distortion amount (level) of the third order distortions 43 and 44 is L-.DELTA.L. When an attenuation amount in the input attenuator 16 is ATT, as shown in FIG. 3B, the level of the fundamental waves 41 and 42 at the output side of the input attenuator 16 is L-ATT and the level of the third order distortions 43 and 44 is L-(.DELTA.L+ATT). In a spectrum analyzer 15, it is clearly stated in the specifications that when a fundamental wave having a level of X dBm is inputted to the mixer 17, a third order distortion of (X-Y) dBm is generated. From a generation characteristic (a generation principle) of a third order distortion, when the input fundamental wave level is X+.DELTA.X, the third order distortion level is Y+.DELTA.Y, where .DELTA.Y=3.DELTA.X. That is, a third order distortion generated in the mixer 17 is increased by three times of the input fundamental wave level increment .DELTA.X, i.e., 3.DELTA.X.
Therefore, when the fundamental wave level is attenuated by .DELTA.ATT in the input attenuator 16, each of the third order distortions is decreased by .DELTA.ATT. However, the third order distortion generated in the mixer 17 is decreased by 3.DELTA.ATT. From such a relationship, when the attenuation amount of the input attenuator 16 is large, the third order distortion generated in the mixer 17 is greatly reduced to be neglected. The input/output characteristic of the mixer 17 for the fundamental waves 41 and 42 is indicated by a linear line 45. When the level of the fundamental waves 41 and 42 is small and the third order distortion generated in the mixer 17 is in the range to be neglected, the input/output characteristic of the mixer 17 for the third order distortions 43 and 44 inputted to the mixer 17 is indicated by a linear line 46 whose level is lower by .DELTA.L than the characteristic line 45 of the fundamental waves 41 and 42. However, when the level of the fundamental waves 41 and 42 is large to some extent, the third order distortion generated in the mixer 17 cannot be neglected and the characteristic of the third order distortion generated in the mixer 17 is indicated by a linear line 47. The third order distortion appearing at the output of the mixer 17 is a sum of an input third order distortion and a third order distortion generated in a mixer. That is, the sum is represented by; EQU 10(.sup.L (.DELTA..sup.L+ATT))/.sup.10 +10.sup.L'/10
As mentioned above, since the third order distortion is represented by a sum of exponential functions, when the input level of the fundamental waves 41 and 42 is smaller than an intersecting point 48 of the linear lines 46 and 47, the level of the input third order distortions 43 and 44 becomes dominant and when the input level of the fundamental waves 41 and 42 is larger than the intersecting point 48, the third order distortion generated in the mixer becomes dominant. That is, in FIG. 4A, the level of the third order distortions 43 and 44 from the object 11 is dominant in the input level range W.sub.1 while the level of the third order distortion generated in the mixer 17 is dominant in the input level range W.sub.2 whose input level is larger than the input level in the range W.sub.1.
Therefore, in the state where no influence by the third order distortion generated in the mixer 17 exists by making the attenuation amount of the attenuator 16 of the spectrum analyzer large, the third order distortion generated by the object 11 against the fundamental wave level can be known by measuring the fundamental wave level and the third order distortion.
Further, from the above relationship, an intercept point can be obtained as described below. When the gain of the object 11 is assumed to be 1, the input/output level characteristic for the fundamental waves is indicated by a linear line 45a in FIG. 4B. On the other hand, the characteristic of the third order distortion generated by the object 11 against the fundamental waves is indicated by a linear line 46a. When an input level of an fundamental wave is I.sub.P and the output level is I.sub.01, the linear line 45a is represented by I.sub.01 =I.sub.P. When a third order distortion level is I.sub.03, the linear line 46a is represented by I.sub.03 =3I.sub.P +a. When, in the spectrum analyzer 15, an internal intermodulation, i.e., a fundamental wave level I=I.sub.01 is measured in the range where the third order distortion generated in the mixer 17 can be neglected and an input third order distortion level I.sub.03 is measured, and then I.sub.P and 103 are substituted in the equation of the linear line 46a, a value of the constant a can be obtained. The intersecting point 49 of the lines 45a and 46a is usually called an "intercept point" and the coordinate of the intercept point is given by (I.sub.p1 +.DELTA.L/2, I.sub.01 +.DELTA.L/2), where .DELTA.L is given by .DELTA.L=I.sub.01 -I.sub.03, and I.sub.p1 and I.sub.01 denote values of I.sub.P and I.sub.0 at I.sub.03 =0, and it holds that I.sub.01 =I.sub.p1. Incidentally, the larger the coordinate value of the intercept point is, the smaller the generated third order distortion in the object 11 is. Thus, the input level range to the object 11 can be made wider.
In such a way, in a conventional system, the attenuation amount of the input attenuator is manually changed to judge if there is an influence of a intermodulation by a frequency mixer in a spectrum analyzer and a intermodulation distortion is measured. And thus, a intermodulation distortion cannot be automatically measured.
That is, an input attenuator 16 is for making an input signal to be a desired level range and a usual spectrum analyzer is arranged such that the attenuation amount of the input attenuator can be changed in 10 dB steps. Such a magnitude of a step change is enough for the usual spectrum analyzer. In a conventional system, if a third order distortion level is not changed when an attenuation amount of the attenuator 16 is changed by 10 dB, the level is used as a level of the input third order distortions 43 and 44. Then, a difference from an input fundamental wave level is judged as a level difference .DELTA.L between an input fundamental wave and an input third order distortion. When, in such a way, an attenuation amount is changed in 10 dB steps, only one or two measuring points which are not influenced by an internal third order distortion (a third order distortion of a mixer) are available. Further, it is difficult to make sure that these measuring points are really not influenced by an internal third order distortion, and thus a third order distortion level may not sometimes be accurately measured.
In addition, a resolution band width, i.e., a change of band width of each of the band pass filters 19 and 24 is set manually. Further, this setting and a setting of an input attenuation amount are performed in trial and error. Since these band width setting and attenuation amount setting influence each other in displaying spectrums, it is very hard to set a proper resolution band width RBW.
Further, in a conventional system, in order to know a noise level as accurately as possible, a band width VBW of the low pass filter 27 is set to a small value. On the other hand, the measuring time, i.e., the frequency sweeping time T.sub.s can be represented by the following formula when a frequency span (a frequency interval for measurement) is S.sub.pan (Hz). EQU T.sub.s =S.sub.pan (Hz)/{RBW(Hz).times.min(RBW, RBW)(Hz).times.0.5}(sec) (1)
In this case, min(RBW, VBW) indicates smaller one of RBW and VBW. As mentioned above, since VBW is set to a degree of VBW=RBW/10, the measuring time T.sub.s is T.sub.s =10.times.S.sub.pan /{(RBW).sup.2.times.0.5} (sec). Therefore, there is a problem that the measuring time is relatively long.
Further, since a spectrum is displayed as shown in FIG. 2A, a noise is displayed at one frequency point f.sub.N. Although the noise level fluctuates up and down, the noise level is displayed as an approximately constant level due to an integral by the low pass filter 27 and a display integral effect by the display unit 35. Therefore, the noise level changing at random cannot be known.
In a measurement using a spectrum analyzer, when a wave form is displayed in a frequency region, RBW is usually set in trial and error.
Parameters other than RBW, i.e., a band width VBW of the low pass filter 27, a central frequency on a display screen and a frequency range (a frequency span) to be displayed on the display screen are also set manually. In such a way, in a conventional system, various parameters in the spectrum analyzer are set manually. Particularly, a band width RBW by which the resolution is determined is set by setting various parameters in trial and error. Therefore, it takes relatively a long time for the setting and the operation is troublesome. In addition, in a conventional system, an S/N has not been measured by a spectrum analyzer and therefore, an S/N measurement has been desired.